Administration sets for the delivery of medical fluids from a fluid source to a patient typically include a drip chamber for determining flow rate and means such as a roller clamp to control the flow rate. In an intravenous administration set the drip chamber is commonly made of a flexible plastic material. The liquid to be delivered, such as for example, dextrose solution or saline solution, enters the upstream end of the drip chamber. Drops of the liquid are formed at an orifice in the drip chamber and fall to the bottom of the drip chamber, from there proceeding downwstream through the administration set into the patient. The rate at which the drops are formed are typically counted by a nurse or other medical personnel who, given the conversion factor of the number of drops per milliliter for the particular administration set, determines the volumetric flow rate of the medical fluid.
A drip chamber works by utilizing the air pressure within the drip chamber. As the liquid leaves the downstream end of the drip chamber, the volume of air remaining expands, thereby decreasing the air pressure within the drip chamber and permitting an additional drop to fall. The rate of flow out of the drip chamber is determined by a clamp or other means downstream from the drip chamber. Before the administration set is attached to the patient, the set must be primed. This is performed by squeezing and then releasing the walls of the drip chamber, drawing liquid from the solution container into the drip chamber.
A device commonly employed in-line with such an administration set is a burette for measuring specific quantities of liquid. A burette may be used when, for example, the volume to be infused into the patient is less than the volume in the supply container or when two different liquids are desired to be mixed in the burette before delivery to the patient.
In order to prevent air from entering the administration set conduit and perhaps causing an air embolism in the patient, a membrane valve is commonly placed at the bottom of the burette. Typically, this membrane valve is a hydrophilic filter which allows the liquid to pass from the burette through the filter and from there downstream through the administration set but which does not permit the passage of air from the burette. Thus, if all of the liquid in the burette passes downstream of the burette before medical personnel return to the patient's bedside, the membrane valve will prevent air in the burette from passing downstream.
Such membrane valves are usually not as structurally resistant to pressure as would be desired. A problem encountered in using such a membrane valve occurs upon priming of the administration set, discussed above. When the drip chamber downstream of the burette with membrane valve is primed, the resulting pressure from the air being forced out of the drip chamber may rupture the membrane valve.
One solution to this problem has been a procedure with the acronym O.S.C.A.R., formed from the words "Open Squeeze Close and Release". In this procedure the administration set is primed by opening the roller clamp downstream of the drip chamber (Open). Then the drip chamber itself is squeezed (Squeeze). With one hand retaining the drip chamber wall in the compressed position, the roller clamp is then closed with the other hand (Close), whereupon the drip chamber is released (Release). The drip chamber wall thereby regains its original expanded position and liquid is drawn into the drip chamber to prime the set. The roller clamp must be in the closed position during expansion of the drip chamber in order to create the necessary suction of liquid through the membrane valve. With the OSCAR procedure the increased pressure formed during drip chamber compression is relieved downstream of the membrane valve and rupture of the membrane valve is avoided.
The principal problems with this procedure are that the attending nurse may forget to use the OSCAR safeguard steps or not know that the steps are required. Also, the procedure takes additional time to perform and requires both hands. Stated differently, the OSCAR safeguard steps are time-consuming and cumbersome, not part of the usual priming technique for the majority of sets which do not have burettes, yet critical to avoid membrane valve rupture and the possibility of an air embolism which may occur if medical personnel forget to return to the patient before the burette is emptied.
One attempt to prevent membrane valve rupture while eliminating the need for OSCAR is shown is U.S. Pat. Nos. 3,967,620 and 4,056,100. The membrane valve disclosed therein is allowed to lift upwardly from its resting position to allow the passage of air around the membrane valve during compression of the drip chamber wall. This construction may be rather expensive and problems are possible with the positioning of the membrane valve itself.
FIG. 5 of the '620 patent discloses an alternate embodiment whereby a check valve is employed comprising a tubing segment having a slit therein, the tubing segment communicating between the drip chamber and the burette such that upon squeezing of the drip chamber air is forced through the slit into the burette chamber, reducing pressure on the membrane valve. Such an embodiment avoids problems with the positioning of the membrane valve but appears to be hard to manufacture consistently. In addition, it is important that upon release of the drip chamber wall that liquid or air does not pass from the burette through the check valve slit into the drip chamber. This may be difficult to avoid due to head pressure from the liquid supply which is greater than atmospheric pressure.
A further attempt to avoid membrane valve rupture is disclosed in U.S. Pat. No. 4,198,971. In the device of that disclosure an air filter 94 allows air in the drip chamber to communicate with the ambient atmosphere upon selective engagement of a rotatably mounted vent cap 82 (FIG. 5), an elastic sealing band 110 (FIG. 6) or a slide clamp 128 (FIG. 7). Selectively positioning the vent cap, sealing band or slide clamp requires an additional step in the priming procedure and appears to be an alternate means of performing OSCAR. Additionally, these selective closures may accidently be left in the open position. Indeed, the devices are meant to be operated at times in the open position to allow air to enter from the atmosphere into the drip chamber, serving as a means to lower the level of liquid in the drip chamber if same becomes flooded. Although the filter 94 is intended to remove microorganisms from the air entering the drip chamber, it does provide for communication of the atmospheric air into the set pathway. When a flexible solution container is used the need for an air vent for the solution container itself is eliminated and the only opening in the pathway is a filtered air vent in the burette. The purpose of the burette vent is not to draw in air from the atmosphere however, and a closed system is possible.
An additional problem which may be present with the device disclosed in the '971 patent is finding a filter element 94 with pore sizes small enough to function as a superior eliminator of microorganisms from the atmosphere while still allowng air to exit quickly from the drip chamber upon the compression cycle of priming to avoid rupturing the membrane valve.
The device of the present invention prevents rupture of the membrane valve, eliminates the need for the OSCAR safeguard steps and quickly and selectively permits the expulsion of air from the drip chamber, while preventing the influx into the fluid pathway of air external to the administration set during either priming or set operation, thereby maintaining what may be a closed system.